Photomultiplier tube circuit for a color analyzer



April 1954 F. E. TOWNSEND 3,130,316

PHOTOMULTIPLIER TUBE CIRCUIT FOR A COLOR ANALYZER Filed Sept. 15, 1960 3 Sheets-Sheet 2 INVEN TOR. F/eAA/cfs E. Rama/sewn BYW/A W Apnl 21, 1964 F, E. TOWNSEND 3,130,316

PHOTOMULTIPLIER TUBE CIRCUIT FOR A COLOR ANALYZER Filed Sept. 15, 1960 3 Sheets-Sheet 3 FILE-.5

INVENTOR. FeANcl's 6'; Fawn/sew.)

A TTQQNEY 3,130,316 1C6 Patented Apr. 21, 1964 3,130,316 PHQTOMULTHLHER TUBE QIRCUIT FOR A CULQR ANALYZER Francis E. Townsend, 1305 N. Hudson, Oklahoma City, Okla.

Filed Sept. 15, 1%(3, Ser. No. 56,200 8 Claims. (Cl. 250-207) This invention relates generally, as indicated, to improvements in color analyzers. Although the apparatus of this invention has utility in various industrial applications, it is particularly useful in processing color negatives in the photographic field and will be described in detail as applied to that application.

As it is well known in the art, it is extremely difficult to make a properly color-balanced print from a color negative. Although the paper on which prints are made from color negatives normally sets forth the various settings of the photographic printer for the particular paper, aging of the paper and variations in operating conditions render the data on the paper only approximate. In the time-honored method of color photography printing, a plurality of prints are made-with variations of the settings of the printer by trial and error until an acceptably color-balanced print is produced. Obviously, such a trial-and-error procedure is only as efficient as the operator of the printer and is normally time-consuming, as Well as expensive.

Various types of color analyzers have been proposed to facilitate the printing of a properly color-balanced print. In the main, prior color analyzers have been overly expensive and extremely difiicult to operate.

All, or practically all, color analyzers utilize a photomultiplier tube having a cathode, an anode and a plurality of intermediate electrodes for measuring the intensity of a light beam directed onto the cathode of the tube. The sensitivity of the photomultiplier tube changes quite widely as a function of the wave lengths of the light impinging on the cathode, such that different voltages must be impressed on the tube for measuring different colors of light. Prior color analyzers have utilized a sliding wire type of variable resistor arrangement for adjusting the voltages impressed on a photomultiplier tube which does not provide sufiicient precision for a truly accurate measurement. In this same connection,

' it may be noted that it is virtually impossible to set a sliding wire type of variable resistor to a given value, such that the setting of the variable resistor can not be reproduced, as when an analyzer has been used in a processing operation for one type of printing paper and is then re-used for processing prints on a different kind of printing paper. In other words, when an operator changes from one type of printing paper to another, the sensitivity of the analyzer must be adjusted, and if the voltages impressed on the photomultiplier tube can be adjusted precisely to a given value, substantial time can be saved in a processing operation.

Another problem which has plagued prior workers in the art is accuracy in registering the output of the photomultiplier tube. The voltage produced by a photomultiplier tube is relatively small, yet will vary over an appreciable range depending upon the variations in intensity of the light beams impinging on the cathode of the tube. Most prior workers in the art have utilized the output of the photomultiplier tube to charge a condenser, from i which a reading of the sensitivity of the photomultiplier tube can be obtained. This type of readout circuit requires a multiplicity of components, variations in the operation of each of which varies the accuracy of the final reading. Also, this type of readout circuit is particularly sensitive to variations in line voltage utilized for energizing the circuit.

The present invention contemplates a novel color analyzer utilizing a photomultiplier tube wherein voltages are impressed across the photomultiplier tube by means of What may be considered a digital type, variable resistance circuit. This variable resistance circuit comprises four channels for selectively impressing voltages across the photomultiplier tube and each channel includes three stepping-type potentiometers (analogous to stepping-type switches) such that a predetermined voltage may be precisely impressed across the photomultiplier tube and, once the desired voltage is obtained and recorded, it may be reproduced at any later date. The circuit for impressing voltages across the photomultiplier tube also includes a novel power supply circuit having a series of voltage regulator tubes for selectively connecting the photomultiplier tube to the power supply at various potential levels, such that photomultiplier tubes of varying characteristics can be easily substituted in the circuit.

The present color analyzer also contemplates a novel read-out circuit for registering the output of the photomultiplier tube with the ultimate in precision. The preferred read-out circuit utilizes a pair of triodes connected in opposing relation to balance out variations in the operating characteristics of the circuit components. The output of the photomultiplier tube is fed to the grid of one of the triodes and a suitable registering means, such as a meter, is connected across the cathodes of the triodes such that the triode receiving the output of the photomultiplier tube acts as a power amplifier and provides a reading on the registering means which is easily discernible. Another novel feature of the readout circuit is the provision of a voltage divider in the power supply circuit between the plates and cathodes of the triodes and wherein the grids of the triodes are tapped to a central portion of the voltage divider to maintain the maximum stability for the triodes and provide the maximum linearity in the desired operating range of the triode acting as a power amplifier. It may also be noted here that suflicient resistance is used in the output of the photomultiplier tube to provide the desired voltage at the input of the triode used as a power amplifier.

The present invention further contemplates a novel probe construction for selectively measuring the intensity of light passing through any desired portion of a color negative. This probe generally comprises a housing having the photomultiplier tube supported therein and having an aperture in one wall of the housing aligned with the cathode of the photomultiplier tube for directing a light beam from any desired portion of a color negative onto the cathode. In a preferred embodiment, a plate or layer of diffusion material is applied across the lower surface of the aperture for directing a uniform amount of light onto the cathode, regardless of the angle of incidence of the light with respect to the aperture. The probe also includes a filter disc supported between the aperture and cathode of the photomultiplier tube having primary color filters therein for the maximum convenience in measuring the intensity of any desired color passing through a predetermined and finite portion of a color negative.

An important object of this invention is to provide a simple and convenient means for accurately measuring color.

Another object of this invention is to provide a novel color analyzer utilizing a photomultiplier tube; wherein a predetermined voltage may be impressed across the photomultiplier aube and such voltage may be reproduced at any time.

Another object of this invention is to provide a novel color analyzer which may be easily reset when changing from one type of printing paper to another in a color photograph printing operation.

A further object of this invention is to provide a novel color analyzer utilizing a photomultiplier tube and wherein the maximum stability is attained-both in the circuit used for impressing a voltage across the photomultiplier tube and in the circuit used for registering the output of the photomultiplier. A related object is to provide such circuits wherein the various components may be replaced and normal variations in the manufacture of these circuit components will not afiect the accuracy or stability of the circuits.

Another object of this. invention is to provide a novel color analyzer wherein the read-out circuit for the photomultiplier tube has the maximum stability and linearity.

A still further object of this invent-ion is to provide a novel probe for a color analyzer wherein a precisely-sized light beam will be impinged on the cathode of the photomultiplier tube of the analyzer, regardless of the angle of incidence of the light with respect to the probe.

Another, and more general, object of this invention is to facilitate the processing of color photographs.

Another object of this invention is to provide a novel color analyzer which is simple in construction, may be economically manufactured and will have a long service life.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawings which illustrate my invention.

In the drawings:

FIGURE 1 is a block diagram of a color analyzer constructed in accordance with this invention.

FIGURE 2 is a schematic wiring diagram of a preferred color analyzer.

FIGURE 3 is a wiring diagram of a typical digital type, variable resistor channel used for impressing a predetermined voltage across the photomultiplier tube.

FIGURE 4 is a vertical sectional view through a preferred probe construction, with portions of the apparatus shown schematically to more clearly illustrate the invention.

FIGURE 5 is an enlarged sectional view through the aperture of the probe to illustrate details of construction.

FIGURE 6 is a sectional view as taken along lines 66 of FIG. 4.

The block diagram shown in FIG. 1 is provided to illustrate the general portions of a color analyzer constructed in accordance with this invention. A suitable power supply 10, which will usually be line voltage, is supplied to a low voltage section 12 and a high voltage section 14. Both of the sections 12 and 14 are provided with suitable circuitry, as will be described in detail below, to rectify the alternating current supplied from the source and to provide a regulated DC. voltage output. The high voltage DC. is fed through a four-channel voltage attenuator 16 for the application of the desired potential across a photomultiplier tube of a photo tube and filter selection portion 18. The portion 18 of the analyzer is provided to sense the intensity of a light beam directed thereto and the light beam may be filtered through any desired color filters, such that the intensity of any desired color may be sensed by the analyzer. The output of the analyzer portion 18 is fed to a read-out circuit 20 having a lower DC. voltage impressed thereon from the low voltage section 12 to amplify the output of the analyzer portion 18 and provide a discernible and accurate reading of the response to the photomultiplier tube of the portion 18.

FIGURE 2 is a more detailed wiring diagram of the various portions of the color analyzer illustrated by the block diagram of FIG. 1. As shown in FIG. 2, A.C. line voltage is impressed across the primary 22 of a suitable transformer 24 and through a suitable switch 26 by means of which power may be supplied to or removed from the transformer 24. In a preferred embodiment, secondaries 28 and 30 of the low and high voltage sections 12 and 14, respectively, are associated with the transformer 24 to energize the low and high voltage sections from the common power source It), although it will be understood that power may be supplied separately to the sections 12 and 14.

Current induced in the secondary 34 of the transformer 24 is rectified by a suitable rectifier 32 and used to charge a suitable condenser 34. The charge produced on the condenser 34 is fed through a suitable resistor 36 and a series of high-voltage, voltage regulator tubes 33. A movable contact 4b is arranged in any desired manner with respect to the voltage regulator tubes 38 for selective contact with the voltage regulator tube circuit between any adjacent pair of the voltage regulator tubes to supply the desired potential to the four-channel voltage attenuator circuit 116. In a preferred embodiment, the voltage regulator tubes 33 are arranged around the arc of a circle, and the movable contact 4d is arranged in the nature of a pivotable arm to contact suitable stationary contacts located between any adjacent pair of the voltage regulator tubes 33. As it is well known in the art, photomultiplier tubes vary in their operating characteristics and in the requirements for the amount of voltage to be impressed to attain the maximum sensitivity. The use of the voltage regulator tubes 33 connected in series, in cooperation with the movable contact 4%), greatly facilitates the adjustment of the voltage to be impressed across a photomultiplier tube and allows the use of a wide range of photomultiplier tubes in the present analyzer. It will also be noted that the power supplied to the four-channel voltage attenuator 16 is automatically regulated by the voltage regulator tubes 38 to provide a substantially constant power supply at any setting of the contact 40.

The attenuator 16 comprises four variable resistance channels 42, 44-, 4-6 and 48 connected in parallel to the movable contact 4t for selectively impressing a voltage across the photomultiplier tube 50 of the photo tube and filter selection portion 18 of the analyzer. The variable resistance channels 42, 44, 46 and 48 will be described in detail below and are connected to stationary contacts of a turret-type switch 52 having a movable contact 54. It may also be noted that the switch 52 has a fifth contact 56 which is not connected to any other portion of a circuit and forms an Off position for the movable contact 54. The movable contact 54 is in turn connected to the voltage divider 53 of the photomultiplier tube 5t).

The tube 5%} may be of any desired type and manufacture, such as a 931A type of tube, and comprises a glass vessel 69, a photosensitive cathode 62, an anode 64 and a series of intermediate electrodes 66 positioned in spaced relation from one another between the cathode 62 and anode 64-. The cathode 62 is the most negative of all elements within the multiplier tube and a voltage of, for example, volts is impressed between this cathode and next electrode 66, as well as between subsequent electrodes in such a way that the electrodes become increasingly more positive, with the anode 64 being the most positive of all. In this manner, the few electrons which are emitted from the photosensitive cathode upon exposure to light are attracted by the adjacent electrode 66 Where they cause the emission of secondary electrons. The number of these secondary electrons is larger than the number of the primary electrons, and the ability of the tube to multiply electrons is based on this fact. The secondary electrons emitted by the electrode 66 adjacent the cathode 62 are in turn attracted to the next electrode 66 where they cause the emission of still more tertiary electrons. This process is repeated in each stage so that finally a fairly heavy current flows between the anode 64 and the adjacent electrode 66. As will be noted in the circuit diagram, the various electrodes 66 and the cathode 62 are connected to corresponding taps of the voltage divider $8, with the cathode 62 being connected directly to the movable contact 54 of the switch 52. The electrode 66 adjacent the anode 64 is connected through the last resistor of the voltage divider 58 to ground, such that the cathode 62 will be the most negative electrode element F in the tube 50, and the electrodes 66 will be progressively more positive to induce the above-described current t6l1rough the tube 50 from the cathode 62' to the anode To facilitate the operation of the present analyzer, the common ground of the analyzer is connected to the movable contact 69 of another turret-type switch 79 having five stationary contacts. One of the stationary contacts 72 of the switch 70 is what may be considered dead to establish an Off position for the switch 711, and each of the other stationary contacts of the switch 71) are connected through suitable lamps '74 to another secondary Winding 76 of the power supply transformer 24. As will be described in more detail below, the movable contact 68 of the switch 71 is mounted on the same shaft as the movable contact 54 of the switch 52, such that each of the lamps 74 will be associated with one of the channels of the attenuator 16 to indicate which channel is being utilized for impressing voltage across the photomultiplier tube 50. The lamps 74 are, of course, connected in parallel and may in turn be connected in series with a pair of lamps 78, such that the lamps 78 will indicate whenever any channel of the attenuator 16 is being used.

The output of the photomultiplier tube 50 is in the form of a DC. current variation, depending upon the intensity of the light beam impinging on the cathode 62 and the sensitivity of the tube 51). As previously indicated, this output is fed to the read-out circuit 21! which is illustrated in some detail in the upper portion of FIG. 2.

The read-out circuit 2t) comprises a pair of triodes 82 and 84 having like characteristics. In a preferred embodiment of this invention, the triodes 82 and 84 are of the same type and produced by the same manufacturer to minimize variations in the characteristics thereof. However, I have found that as long as the triodes 82 and 84 are of the same types their characteristics are sufiiciently similar to provide an operable read-out circuit. The plates 86 and 88 of the triodes 82 and 84 are connected to one end of a voltage divider comprising a pair of resistors 90 and 92'; whereas the cathodes 94 and 96 of the triodes 82 and 84 are connected (as will be described in detail below) to the opposite end of the voltage divider comprising the resistors 90 and 92. The remote ends of the resistors 9t9and 92 are connected across a suitable low-voltage, voltage regulator tube 98. The tube 98 is in turn connected through a limiting resistor 99 across a condenser 1110 being charged from the secondary winding 28 through a rectifier 1132, such that the voltage produced across the condenser is impressed across the plates and cathodes of the triodes 82 and 84.

The grids 104 and 1116 of the triodes 82 and 84 are connected through resistors 1118 and 110 to the center point of the voltage divider comprising the resistors 90 and 92. Therefore, the potential applied on the grids 104 and 166 is intermediate the potential supplied to the plates and cathodes of the respective triodes to increase the stability of the triodes. In a preferred embodiment, the resistors 90 and 92 are of equal resistance values, such that the potential applied to each of the grids 104 and 106 is one-half of the difference in potential applied to the plates and cathodes of the respective tubes. It will also be noted that the anode 64 of the photomultiplier tube 50 is connected to the grid 1114 of the triode 82 between the resistor 108 and the respective grid, such that the output of the photomultiplier tube is directed through a substantial resistance and the desired voltage is applied to the grid 1%.

The triodes 82 and 84 are cathode-follower types and the cathodes thereof are connected to the opposite ends of a variable resistor 112 having its movable contact 114 connected to the negative side of the condenser 100. The response of the readout circuit is obtained by a suitable meter 116 connected through a current-limiting resistor 118 to the cathodes of the triodes 32 and S4 in parallel with respect to the variable resistor 112.

Upon examination of the arrangement of the triodes 82 and 84, it will be apparent that any variation in the operation of one of the triodes is balanced out by a similar variation in the opposite triode to overcome any variations in the ideal of tube construction. Also, since the grids 104 and 106 are normally retained at fixed potentials with respect to the potentials of the plates and cathodes, and since there is a substantial dilference in potential between the grids and the plates and cathodes, the triodes will be operated in their most desirable ranges to obtain the maximum in stability and linearity. The current passed through the triodes 82 and 84 can be easily adjusted by the movable contact 114 of the variable resistor 112 to easily zero the meter 116 when the same potential is applied to both of the grids 104 and 106.

As previously indicated, the sensitivity of the photomultiplier tube 50 varies with the color of light impinging upon the cathode 62 thereof; and the output of the photo-. multiplier tube, when amplified by the triode 82, must be within the range of the meter 116. Thus, the various channels of the attenuator 16 are provided to adjust and select the magnitude of the voltage impressed across the photomultiplier tube. Each of these channels is constructed in the same manner, and a typical construction is illustrated by the wiring diagram in FIG. 3. Each channel comprises three stepping-type potentiometers 118, 121) and 122 connected in series to provided a digital-type voltage selection. Each of the stepping-type potentiometers comprises a plurality of resistors 124 (preferably nine) connected in series and arranged on the arc of a circle with stationary contacts 126 between each adjacent pair of resistors. A movable contact 128 is pivotally supported centrally with respect to the respective resistors 124 to selectively engage the stationary contacts 126 and vary the total resistance provided by the respective potentiometer. In a preferred embodiment, each resistor 124 of the potentiometer 118 has a value corresponding to the resistance value of each of the resistors in the voltage divider 58 of the photomultiplier tube 50; each resistor 124 of each potentiometer has a resistance value of one-tenth of the resistance value of each of the resistors in the potentiometers 113; each resistor 124 of the potentiometers 122 has one-tenth of the value of each resistor 124 in the respective potentiometer 121); and each potentiometer has nine resistors therein. With this preferred arrangement, each channel of the attenuator 16 can be precisely set to provide a predetermined voltage across the photomultiplier tube 50, and the voltage impressed across the photomultiplier tube by any particular channel may be varied in steps of one percent of the resistance between adjacent electrodes of the photomultiplier tube to provide the ultimate in precision.

When the present analyzer is being used in the processing of color negatives, the photomultiplier 50 (see FIG 4) is mounted in a housing 130 which forms what may be considered a probe for receiving a light beam projected through any preselected portion of the color negative being processed. The housing 139 is of any desired configuration, such as box-shaped, and an aperture 132 is formed through the top wall 134 of the housing above the photomultiplier tube 51). It will be understood that the photomultiplier tube 50 is secured in a suitable socket 136 in the housing 130 with the cathode 62 thereof in alignment with the aperture 132. The walls of the aperture 132 (FIG. 5) are preferably tapered downwardly and outwardly from the upper end of the aperture (which is the end opposite the cathode of the photomultiplier tube), such that a light beam impinging on the aperture 132 from an angle other than the vertical will not be substantially restricted by the walls of the aperture. Also, a plate of diffusion material is secured across the lower end of of the aperture 132, preferably in contact with the lower face of the wall 134, to direct light passing through the aperture 132 onto the cathode 62. The plate of diffusion material 138 will distribute light onto the cathode 62 if placed anywhere between the aperture 132 and the cathode, as long as the diffusion material is in the path of a light beam passing through the aperture. However, when the diffusion material is applied. directly over the lower end of the aperture, a light beam having only the diameter of the aperture will be directed onto a desired portion of the cathode 62 to provide the maximum response for the photomultiplier tube. It will be apparent that the diffusion material acts as a light collecting and control device to effectively change the direction of light passing through the aperture 132. In a commercial embodiment of the probe, the wall 134 is formed of 24- gage metal and the diameter of the aperture is between .050 and .060 inch, which 1 have found particularly useful in providing the maximum sensitivity for the photomultiplier tube.

A shaft 140 (FIG. 4) is suitably journaled in a vertical position in the housing 130 adjacent the free end of the photomultiplier tube 519 by means of suitable bearings 1 2-2 and has a filter wheel 144 rigidly mounted thereon in such a position that one edge of the filter wheel extends between the aperture 132 and the photomultiplier tube cathode 62. The filter wheel 144 is a circular plate having any desired colors of filters therein arranged in a circular pattern, as illustrated in FIG. 6. In a preferred embodiment, the filter wheel 144 contains a red filter 146, a green filter 148, a blue filter 156), an open space or aperture 152 and either a blank or open space 154 arranged in equallyspaced circumferential relation around the filter wheel. Also, the movable contact 54 of the switch 52 and the movable contact 68 of the switch 70 are mounted on the shaft 140 for movement with the filter wheel 144. It will thus be apparent that the various filters in the filter wheel 144 will be associated with the various stationary contacts of the switches 52 and 71) to place the desired filter between the aperture 132 and the cathode 62 when the associated stationary contacts of the switches 52 and 70 are engaged. For example, the red filter 146 may be in operating position between the aperture 132 and the cathode 62 when the channel 48 of the attenuator 16 is engaged by the movable contact 54 and while an associated stationary contact of the switch 70 is engaged by the movable contact 68 to illuminate an associated one of the lamps 74. With this arrangement, the channel 46 of the attenuator is associated with the green filter 148; the channel 44 of the attenuator is associated with the blue filter 150; the channel 42 of the attenuator is associated with the open space 152, and the blank or no-filter position 154 is associated with the Off positions of both of the switches 52 and 71).

A suitable knob 156 is provided on the upper end of the shaft 140 and may be used in cooperation with indicating marks (not shown) on the upper face of the housing 130 to facilitate the positioning of the shaft 140 and operation of the analyzer.

Operation Preparatory to the printing of one or more color negatives having an unknown color balance, the apparatus of this invention must be adjusted with respect to a known standard which is normally known as a master negative. The master negative must contain areas similar to one or more areas of the negative or negatives having the unknown color balance. As it is well known in the art, a master negative will normally contain a gray card, a white card, a skin-tone, a density step wedge and a color patch card.

When the proper master negative has been selected, a perfectly color-balanced print is made from the master negative, whereby the total filter pack of the printer (not shown) being used becomes the basic filter pack for that particular master negative and printing material. Some trial and error may be necessary to obtain the perfectly color-balanced print from the master negative.

With all printing conditions the same as when the perfectly color-balanced print was made from the master negative, the probe 18 is placed on or in the printer until the aperture 132 is aligned with an area of the master negative which is common to areas of the unknown colorbalanced negatives. The control knob 156 of the probe 13 is then turned until the movable contact 54 of the switch 52 is aligned with what may be called the white channel 42 of the attenuator 16 and the movable contact 68 is aligned with the corresponding contact of the switch '74 It will be recalled that with the switches 52 and 70 in these positions, the open aperture 152 of the filter wheel 144- is aligned with the aperture 132, such that the cathode 62 of the photomultiplier tube 56 will be exposed to all of the light passing through the aperture 132. With this arrangement of the probe 13, the deflection of the meter 116 will indicate the light intensity, and hence the duration of exposure used on the master print. It will be understood that the face of the meter 116 may be calibrated in time as well as in suitable units of light intensity. The potentiometers 118, 1211 and 122 of the white channel 42 are then adjusted until the needle of the meter 116 coincides with the exposure time used for making the perfectly colorbalanced print from the master negative. This setting of the white channel 42 is suitably recorded by the operator of the printer.

The operator next turns the switches 52 and 70 until the movable contact 54 is aligned with the channel 48 of the attenuator 16; the red filter 146 of the filter wheel is aligned with the aperture 132; and the movable contact 68 of the switch 70 is aligned with the appropriate stationary contact. Therefore, the channel 48 may be considered as the red channel of the attenuator. With this arrangement, the various stepping-type potentiometers 118, and 122 of the channel 18 are adjusted until the needle of the meter 116 coincides with a suitable index marker (not shown) on the face of the meter. It will be understood that this index marker on the meter 116 will be in the central portion of the range of the meter. The same procedure is followed with respect to the green and blue channels 46 and 44 with the green and blue filters 148 and 150 aligned with the aperture in the probe 18. The apparatus of this invention will then be adjusted to the desired control area of the master negative.

The master negative is then removed from the printer; all settings of the color analyzer are recorded, and the unknown color-balanced negative is placed in the printer. With the aperture 132 of the probe 18 aligned with an area of the unknown color negative which is common to the control area of the master negative used, the knob 156 of the probe is turned to set the movable contact 54 of the switch 52 in contact with the red channel 48; whereupon the diaphragm or iris of the printer is adjusted until the needle of the meter 116 is again on the index mark. The diaphragm is adjusted while the analyzer is set to measure the amount of red light passing through the control area of the unknown color negative, since all present-day printing paper requires predominantly red light for proper exposure. Therefore, when the meter 116 is on the index mark while measuring red light, one is assured that the same setting of the diaphragm can be used to measure the amount of green and blue light passing through the control area of the unknown color negative.

The probe 1 8 is then adjusted to connect the green channel 46 to the photomultiplier tube 50 and place the green filter 143 in alignment with the aperture 132. The reading of the meter 116 will then indicate the amount of green light passing through the control area of the unknown color negative and will indicate whether green or magenta filters are required in the filter pack of the printer, depending upon what type of filters are being used. For example, if the needle of the meter 116 is below the index mark, either the appropriate number of magenta filters are removed from the filter pack, or the appropriate number of green filters are added to adjust the needle to the index mark. On the other hand, if the needle is above the index mark, the appropriate number of filters are added to or removed from the filter pack of the printer to reduce the amount of green light and align the needle with the index mark. The analyzer is then adjusted to connect the blue channel 44 with the photomultiplier tube 50 and align the blue filter 150 with the aperture 132 of the probe 18. The same procedure is then followed with respect to adjusting the filter pack of the printer to bring the needle of the meter 116 to the index marker. It is then usually the best procedure to recheck the response of the photomultiplier tube 50 to the various channels of the attenuator 16 and filters of the filter wheel 144 to be sure that the needle of the meter 1116 remains on the index marker when the three primary colors are being measured. When the needle of the meter 116 does remain on the index marker while reading each of the primary colors, a color balance is obtained for the particular negative.

The control knob 156 of the probe 18 is then again turned to connect the white channel 42 of the attenuator 16 to the photomultiplier tube 50 and place the open aperture 152 of the filter wheel 144 in alignment with the aperture 132; whereupon the diaphragm of the printer is again adjusted until the needle on the meter 1116 again reads the same as the exposure time which was used for making the print from the master negative. In this connection, it may be noted that adjustment of the diaphragm will not affect the color balance for the particular negative from which prints are desired.

The paper used to make a perfect print from the master negative is then placed in the printer and the unknown negative is exposed for the time indicated to obtain a perfectly color-balanced print.

Since all of the settings of the various channels of the attenuator 16 are recorded during adjustment of the analyzer to a particular negative having a particular control area and to a particular type of printing paper, the analyzer may be easily reset in a minimum of time when a new negative having the same control area is used for making prints on the same kind of printing paper. Thus, the analyzer may be adjusted from one operating condition to another in a minimum of time and does not have to be adjusted by any trial-and-error method when changing from one negative to another or from one printing paper to another.

When considering the operation of the photomultiplier tube 50 and the read-out circuit 20, it will be apparent that the output of the photomultiplier tube is applied to the grid 104 of the triode 32 and is amplified by the triode 82 into the form of current for easy readability by the meter 116. The connection of the triodes 82 and 84 in balancing relation with the same potentials being applied across the plates and cathodes thereof virtually eliminates any variations in the characteristics of the triodes. The maintenance of the grids of the triodes 8-2 and 84 at a potential substantially half-way between the potentials of the plates and cathodes of the triodes maintains the triodes in the central portions of their operating ranges and attains the maximum linearity, even though the potential applied to the triodes from the power source may fluctuate within reasonable limits.

The decade-type of variable resistors of the attenuator 16 not only facilitates the application of the desired voltage on the photomultiplier tube 50 for the particular colors being analyzed, but also permits recording of the voltage being impressed across the photomultiplier tube through any one of the channels. As a result, any channel of the attenuator 1 6 may be reset to a desired reading at any later date. It should further be noted that the movable contact 40 of the attenuator can be connected to any desired portion of the power supply circuit 114 to provide a basic adjustment of the voltage impressed across the photomultiplier tube 50, such that the entire analyzer may be easily adjusted to a wide range of types of photomultiplier tubes.

The probe 18 provides a simple and compact construction wherein the light to be measured is correctly directed onto the cathode 62 of the photomultiplier tube, regardless of the angle of incidence of the light with respect to the aperture 132. Also, with the utilization of the diffuser plate 138 across the lower end of the aperture 132, s-uificient space is provided in the probe housing for the filter wheel 1 44 and switches 52 and 70, such that all portions of the analyzer which should be set at the same time can be manipulated by use of the single knob 156- for easy and efiicient adjustment of the analyzer. From the foregoing, it will be apparent that the present invention provides a simple and convenient means for accurately measuring color. Any predetermined voltage may be impressed across the photomultiplier tube and any desired setting of the voltage-applying circuit may be obtained at a later date to provide the maximum simplicity in processing different color negatives and when using different types of printing paper. The arrangement and combination of circuit components not only provide the maximum simplicity, but attain the maximum stability in the analyzer and allow the replacement of the various circuit components without affecting the accuracy of the analyzer. The read-out circuit is particu larly attractive in obtaining linearity and stability for accurately registering the low magnitude output of the photomultiplier tube. It will further be apparent that the present analyzer is simple in construction, may be economically manufactured and will have a long service life.

Changes may be made in the combination and arrangement of parts or elements as heretofore set forth in the specification and shown in the drawings, it being understood that changes may be made in the precise embodiment disclosed without departing from the spirit and scope of the invention as defined in the following claims.

I claim:

1. A color analyzer comprising a photomultiplier tube having a cathode, anode and a plurality of intermediate electrodes; means for directing the light beam to be measured onto the cathode of the photomultiplier tube; means for registering the response of the photomultiplier tube, and means for impressing a voltage across the photomultiplier tube, the last-mentioned means comprising a source of D.C., at least three sets of variable resistors connected to said source of DC, and a switch for selectively connecting each of said sets to said photomultiplier tube, each set of said variable resistors comprising a plurality of stepping-type potentiometers connected in series, each of said stepping-type potentiometers comprising a series of uniform resistors arranged in a circular pattern, a fixed contact between each pair of uniform resistors, and a switch arm movable to successive fixed contacts.

2. A color analyzer, comprising:

a photomultiplier tube having a cathode, an anode and a plurality of intermediate electrodes;

means for impressing a voltage across the photomultiplier tube;

means for directing the light beam to be measured onto the cathode of the photomultiplier tube;

a pair of triodes of similar characteristics;

a source of DC. connected across the plate and-cathode circuits of both of said triodes;

a voltage divider across said D.C. source, the grids of said triodes being connected to an intermediate portion of said voltage divided for biasing the grids of said triodes to the same potential;

a conductor connected to the anode of the photomultiplier tube and to the grid of one of said triodes to control the conduction of said one triode in response to the output of the photomultiplier tube, and

registering means connected across the cathode circuits of said triodes for registering the amplified output of the photomultiplier tube.

3. A color analyzer, comprising:

a photomultiplier tube having a cathode, an anode and a plurality of intermediate electrodes;

means for directing the light beam to be measured onto the cathode of the photomultiplier tube;

means for registering the response of the photomultiplier tube, and

means for impressing a voltage across the photomultiplier tube circuit,

said means for impressing a voltage across the photomultiplier tube circuit including:

a source of D.C.;

a plurality of voltage regulator tubes connected in series to said source, and

a movable contact connected to the photomultiplier tube circuit for selectively impressing a potential across the photomultiplier tube circuit from between adjacent regulator tubes.

4. A11 analyzer as defined in claim 1 wherein each set of variable resistors comprises three of said steppingtype potentiometers, and wherein the uniform resistors of each set of three stepping-type potentiometers have resistance value ratios of 1, l0 and 190.

5. An analyzer as defined in claim 2 wherein said voltage divider comprises two resistors having substantially equal resistance values, and said grids being connected to said D.C. source between said resistors.

6. An analyzer as defined in claim 2 characterized further to include a variable resistor in the cathode circuits of said triodes in parallel around said registering means and having its movable contact connected to said DC. source for balancing said triodes when no potential is being supplied by said photomultiplier tube.

7. An analyzer as defined in claim 1 wherein said means for impressing a voltage across the photomultiplier tube includes a plurality of voltage regulator tubes connected in series to said source, and a movable contact connected to said sets of variable resistors for selectively impressing a potential across the photomultiplier tube from between adjacent voltage regulator tubes.

8. An analyzer as defined in claim 4 wherein said means for impressing a voltage across the photomultiplier tube includes a resistor between each adjacent pair of electrodes of the photomultiplier tube, and wherein each of said last-mentioned resistors has a resistance value corresponding to said uniform resistances having the highest resistance value.

References Cited in the file of this patent UNITED STATES PATENTS 2,472,380 Long June 7, 1949 2,571,838 Connor et al Oct. 16, 1951 2,740,903 Willcox Apr. 3, 1956 2,774,276 Glasser et al Dec. 18, 1956 2,850,644 Parsons Sept. 2, 1958 3,023,318 Jones Feb. 27, 1962 

3. A COLOR ANALYZER, COMPRISING: A PHOTOMULTIPLIER TUBE HAVING A CATHODE, AN ANODE AND A PLURALITY OF INTERMEDIATE ELECTRODES; MEANS FOR DIRECTING THE LIGHT BEAM TO BE MEASURED ONTO THE CATHODE OF THE PHOTOMULTIPLIER TUBE; MEANS FOR REGISTERING THE RESPONSE OF THE PHOTOMULTIPLIER TUBE, AND MEANS FOR IMPRESSING A VOLTAGE ACROSS THE PHOTOMULTIPLIER TUBE CIRCUIT, SAID MEANS FOR IMPRESSING A VOLTAGE ACROSS THE PHOTOMULTIPLIER TUBE CIRCUIT INCLUDING: A SOURCE OF D.C.; A PLURALITY OF VOLTAGE REGULATOR TUBES CONNECTED IN SERIES TO SAID SOURCE, AND A MOVABLE CONTACT CONNECTED TO THE PHOTOMULTIPLIER TUBE CIRCUIT FOR SELECTIVELY IMPRESSING A POTENTIAL ACROSS THE PHOTOMULTIPLIER TUBE CIRCUIT FROM BETWEEN ADJACENT REGULATOR TUBES. 